Wire ropes, valve springs, suspension springs and PC wires are produced generally by subjecting steel wire rods obtained by hot rolling (hereinafter referred to simply as "wire rods") to cold working such as drawing or cold rolling and further to the thermal refining treatment of quenching and tempering or to bluing treatment. In addition, fine steel wires for steel cords used as reinforcing materials in radial tires for automobiles are produced by subjecting wire rods of about 5.5 mm in diameter after hot rolling and controlled cooling to primary drawing, patenting treatment, secondary drawing and final patenting treatment and then to brass plating and final wet drawing. A plurality of fine steel wires obtained in this manner are further twisted into a twisted steel wire to produce a steel cord.
Generally, productivity and yield are greatly decreased if breakage occurs upon formation of wire rods into steel wires. Accordingly, it is strongly desired that wire rods in the technical fields described above are not liable to breakage during drawing or cold rolling, particularly during wet drawing where severe cold working is conducted for production of steel cords. Similarly, it is required that breakage does not occur during stranding for twisting a plurality of fine steel wires.
In recent years, there is increasing demand for light-weighing of various products such as wire ropes, valve springs, suspension springs, PC wires and steel cords in the background of cost reduction and global environmental problem. Accordingly, steel products for high strength in these uses are actively researched. However, as the strength of steel products is raised, their ductility and toughness are generally lowered thus deteriorating drawing workability, cold workability in rolling and workability in stranding, and they are also rendered liable to fatigue breakage. Accordingly, wire rods serving as stock for the various products described above are required to be excellent particularly in the internal states thereof.
Accordingly, for the purpose of improving drawing and cold workability for wire rods, simultaneously improving workability in stranding of steel wires and further improving fatigue resistance for the products, techniques directed to cleanliness of steel have been developed. For simplicity in the following description, the drawing workability and cold workability in rolling of wire rods and the workability in stranding of steel wires may also be referred to collectively as "cold workability".
For example, the 126th and 127th Nishiyama Memorial Technical Course, pp. 148 to 150 shows the technique of controlling non-metallic inclusions (hereinafter referred to simply as inclusions) to the region of a ternary low-melting composition which readily undergoes plastic deformation during hot rolling, to make them harmless as deformable inclusions.
JP-A 62-99436 discloses steel wherein an inclusion is limited to a less deformable one with a ratio of length (L)/width (d).ltoreq.5, and the average composition of the inclusion comprises SiO.sub.2, 20 to 60%; MnO, 10 to 80%; and either one or both of CaO, 50% or less and MgO, 15% or less.
JP-A 62-99437 discloses steel wherein an inclusion is limited to a less deformable one with a ratio of length (L)/width (d).ltoreq.5, and the average composition of the inclusion comprises SiO.sub.2, 35 to 75%; Al.sub.2 O.sub.3, 30% or less; CaO, 50% or less; and MgO, 25% or less.
The techniques disclosed in JP-A 62-99436 and JP-A 62-99437 are substantially identical to the technical content reported in the above-described Nishiyama Memorial Technical Course in respect of the technical idea of lowering the melting point of inclusions. The techniques proposed in these 2 publications are those wherein the composition of multicomponent inclusions including MnO and MgO is controlled to lower the melting point, and the inclusions are sufficiently drawn during hot rolling and then the inclusions are disrupted and finely dispersed by cooling rolling or drawing whereby cold workability and fatigue resistance are improved.
However, the interfacial energy of inclusions is very small. Accordingly, the inclusions are readily aggregated and agglomerated in the process of from secondary refining such as ladle refining having a gas bubbling or arc reheating process to casting, so they tend to remain as giant inclusions at the stage of continuously casted slabs. Once the giant inclusions are generated, there is the possibility that even if the average composition of inclusions is the same, crystallization of a heterogeneous phase occurs more frequently in the process of solidification in identical inclusions, as shown in FIG. 1. In FIG. 1, the shaded portion is a heterogeneous phase. Accordingly, even in the case of the composition of inclusions proposed in the respective publications described above, that is, in the case where the average composition of inclusions is regulated, if giant inclusions with a heterogeneous composition are crystallized, the regions of giant inclusions with the composition proposed in the publications are soft and thus made small by hot rolling and cold rolling or drawing, but the portions of giant inclusions not having the composition proposed in the publications can remain large, so there is a limit to the improvement of cold workability and fatigue resistance.
On the other hand, the techniques wherein the size and number of rigid inclusions adversely affecting cold workability and further fatigue resistance are specified are disclosed in JP-A 9-125199, JP-A 9-125200, and JP-A 9-209075. However, the techniques proposed in these publications are those wherein, for example, a test specimen taken from a wire rod of 5.5 mm in diameter obtained by hot rolling is dissolved in a specified solution, and its residues i.e. rigid oxide inclusions (hereinafter referred to simply as oxides) are measured for their size and number, whereby the cleanliness of the steel and steel products can be specified for the first time. Accordingly, if facilities for melting steel are different or if the chemical composition of steel is different, steel and steel products having desired high cleanliness cannot necessarily be obtained stably according to the techniques disclosed in the publications described above.